There are known optical scanning devices in which an image can be recorded or read by being scanned with a laser beam. As one of such optical scanning devices, there has been proposed a device employing a semiconductor laser as a light source and a holographic scanner for deflecting a laser beam emitted from the semiconductor laser (see Japanese patent application No. 59-28066, for example).
The optical scanning device of the above type and problems to be solved by the present invention will be described below with reference to FIG. 1.
FIG. 1 shows an image recorder employing an optical scanning device in which a laser beam emitted from a semiconductor laser is deflected by a holographic scanner.
The image recorder includes a semiconductor laser 10, a collimator lens 12, a cylindrical lens 14, plane mirrors 16, 18, 22, a holographic scanner 20, an f.theta. lens 24, plane mirrors 26, 28, a cylindrical lens 30, a photoconductive photosensitive member 32, and light detectors 34, 36.
Actually, the present invention is embodied in the image recorder shown in FIG. 1. A device similar to the image recorder of FIG. 1, but which lacks the light detector 36, suffers the problems to be solved by the invention. Therefore, insofar as the problems are addressed, the presence of the light detector 36 in FIG. 1 is neglected.
The holographic scanner 20 comprises a hologram disc 20A and a motor 20B. The hologram disc 20A is fixed to the shaft of the motor 20B and driven by the motor 20B to rotate in the direction of the arrow.
A plurality of identically shaped diffraction gratings 200 are arranged in an annular pattern on one surface of a transparent circular base plate of the hologram disc 20A.
The diffraction gratings 200 are linear diffraction gratings and optically equivalent to each other, the diffraction gratings 200 being formed as holograms. The terms "hologram disc" and "holographic scanner" are derived from the fact that the diffraction gratings 200 are formed as holograms.
A laser beam emanating from the semiconductor laser 10 is converted by the collimator lens 12 into a parallel-ray beam, which is passed through the cylindrical lens 14 and reflected by the plane mirrors 16, 18 to fall on the diffraction gratings 200 of the hologram disc 20A, thereby producing a diffraction beam. As the hologram disc 20A rotates, the diffraction beam is deflected since the direction of each of the diffraction gratings 200 varies with respect to the incident laser beam. The diffraction beam as deflected is referred to as a "deflected laser beam".
The diffraction beam is then guided by the plane mirror 22, the f.theta. lens 24, the plane mirrors 26, 28, and the cylindrical lens 30 to reach the photosensitive member 32 which is in the form of a belt. The diffraction beam is focused as a spot on the photosensitive member 32 by the focusing action of the f.theta. lens 24 and the cylindrical lenses 14, 30. Upon rotation of the hologram disc 20A, the spot formed on the photosensitive member 32 by the deflected laser beam is linearly displaced on the photosensitive member 32. The spot is cyclically displaced to repeat an optical scanning process each time the laser beam falls on a different diffraction grating. The spot as it scans the photosensitive member follows a straight path 38, which is referred to as a main scanning line. A direction normal to the main scanning line on the photosensitive member 32 is referred to as an auxiliary scanning direction.
The photosensitive member 32 is continuously moved. After the peripheral surface of the photosensitive member 32 has been uniformly charged, it goes to an optical scanning zone in which it is scanned by the deflected laser beam. If the intensity of the laser beam emitted from the semiconductor laser 10 is modified by an image signal representative of an image to be recorded, an electrostatic latent image corresponding to the image to be recorded is formed on the photosensitive member 32. By developing the electrostatic latent image into a visible image, transferring the visible image onto a recording sheet such as of paper, and fixing the image to the sheet, the desired recorded image can be obtained.
The foregoing is an outline of the optical scanning process.
The light detector 24 is employed to detect the deflected laser beam immediately before it scans the photosensitive member 32, so as to align starting points of successive scanning cycles.
As is well known, the wavelength of the laser beam emitted from the semiconductor laser varies as the temperature of the semiconductor laser changes.
In the optical scanning device of the type in which the laser beam from the semiconductor laser is deflected by the holographic scanner, when the wavelength of the laser beam from the semiconductor laser varies, the angle of diffraction of the diffraction beam from the diffraction gratings also varies, and hence the optical scanning zone, i.e., the position of the main scanning line is moved in the auxiliary scanning direction, failing to perform proper optical scanning operation.
The temperature of the semiconductor laser is changed by the temperature of an atmosphere near the semiconductor laser, the Joule heat generated by current passing through the semiconductor laser, etc. In order to stabilize the wavelength of the laser beam, it is necessary to control the temperature of the semiconductor laser. However, it is difficult to directly control the temperature of the semiconductor laser since the semiconductor laser itself is quite small in size. Therefore, it has been customary to indirectly control the temperature of the semiconductor by controlling the temperature of a holder by which the semiconductor is supported.
The semiconductor laser is generally used in a temperature range of from 20.degree. to 50.degree. C. In such a temperature range, the temperature and the wavelength of the laser beam are related generally as indicated by the staircase-like line 3-1 in FIG. 2. The staircase-like line 3-1 is referred to as a characteristic line indicative of the relationship between the temperature and the wavelength. The shape of the characteristic line depends on an individual semiconductor laser and varies from semiconductor laser to semiconductor laser. However, it is generally of a staircase-like shape. As shown in FIG. 2, the characteristic line includes regions such as A, B, C in which the wavelength changes gradually with respect to the temperature change. These regions are referred to as shelf portions, and those between the shelf portions are referred to as step portions in which the wavelength jumps in value. In reality, each of the shelf portions contains more or less irregularities, but such irregularities are neglected as they do not obstruct the optical scanning operation. The width of one shelf portion, i.e., the interval between one step portion and an adjacent step portion, is normally equivalent to a few degrees of temperature. Assuming that the temperature width of the shelf portion B, i.e., the temperature difference (T.sub.U -T.sub.L), is 5 degrees, and a preset temperature T.sub.0 is selected intermediate between these temperatures T.sub.U, T.sub.L, the wavelength of the laser beam emitted from the semiconductor laser can actually be controlled to a constant level by controlling the temperature of the semiconductor laser holder in the vicinity of the preset temperature T.sub.0, e.g., in the range of T.sub.0 .+-.1.degree. C.
Even if the temperature of the semiconductor laser varies from T.sub.L to T.sub.U, the wavelength of the laser beam varies by a small interval of .lambda..sub.U -.lambda..sub.L. Stated otherwise, as long as the temperature of the semiconductor laser varies within one shelf portion, the variation of the main scanning line presents no actual problems.
If the temperature of the semiconductor laser varies beyond T.sub.U or T.sub.L, however, the wavelength of the laser beam changes abruptly to a large extent of (.lambda..sub.2 -.lambda..sub.U) or (.lambda..sub.L -.lambda..sub.1). When such a wavelength change occurs, no proper optical scanning operation can be performed.
The problems to be solved by the present invention are as follows:
If the temperature of the semiconductor laser holder is controlled so as to be close to the preset temperature T.sub.0, the optical scanning operation will not actually be subject to difficulties arising from variations in the temperature of the semiconductor laser. However, this is based on the premise that the characteristic line remains unchanged with time.
The characteristic line actually does not remain unchanged with time, but changes with time as the semiconductor laser is subjected to fatigue. This time-dependent change takes place in two patterns. In the first pattern, the characteristic line is shifted as a whole toward a lower temperature side aong the shelf portions as indicated by the broken lines in FIG. 3(I). In the second pattern, the characteristic line is shifted as a whole toward a higher temperature side along the shelf portions as indicated by the broken lines in FIG. 3(II).
The time-dependent change of the characteristic line varies from semiconductor laser to semiconductor laser. Some semiconductor lasers have their characteristic lines shifted to the higher or lower temperature side. In other semiconductor lasers, the direction in which the characteristic line is shifted varies with time.
Where the preset temperature for temperature control is T.sub.0 in FIG. 3(I), there will be no problem as far as the semiconductor laser is not subjected to fatigue and the characteristic line is as indicated by the solid line 3-1. However, when the characteristic line varies with time toward the broken line 4-1, the wavelength of the emitted laser beam is caused to vary to a large extent even if the temperature of the semiconductor laser is T.sub.0. This holds true when the characteristic line varies with time toward the broken line 4-2 in FIG. 3(II).
Once the characteristic line varies with time as described above, the temperature control which has conventionally been practiced is no longer effective.
Therefore, it is a first object of the present invention to provide a temperature control method capable of coping with time-dependent changes of the characteristic line.
A second object of the present invention is to make is possible to effect the temperature control method for an extended period of time.
A third object of the present invention is to make it possible to carry out the temperature control method reliably.